This invention relates to a d.c. circuit using a chopper for controlling power supplied from a d.c. power source to a d.c. load, and more particularly to improvements in a protective circuit which detects commutation failure of the chopper to disconnect the d.c. load from the d.c. power source to protect the d.c. circuit.
As is commonly known, a chopper is generally composed of a main thyristor, an auxiliary thyristor, and commutation means comprising a commutation reactor and a commutation capacitor. In the chopper, the main thyristor is turned on to permit flow of direct current to a d.c. load from a d.c. power source, and this state is so called a "chopper-on state." The auxiliary thyristor is turned on to turn off the main thyristor by the action of the commutation means thereby prohibiting flow of direct current to the d.c. load from the d.c. power source, and this state is so called a "chopper-off state."
In the description which follows, a "chopper-on signal" refers to a signal which turns on the main thyristor, and a "chopper-off signal" refers to a signal which turns on the auxiliary thyristor.
On-off control of such a chopper is disclosed in, for example, U.S. Pat. Nos. 3,903,465 and 3,914,672.
However, commutation failure is an inevitable problem for a chopper of this kind in view of the structural principle of the same. That is, commutation failure occurs when the auxiliary thyristor fails to turn on for some reason in spite of the fact that the chopper-off signal has been applied to the auxiliary thyristor for turning off the main thyristor from its conducting state, or when the commutation energy accumulating in the commutation means is not enough to turn off the main thyristor in spite of the fact that the auxiliary thyristor has been turned on in response to the application of the chopper-off signal thereto.
Such commutation failure results in continuous flow of direct current to the d.c. load from the d.c. power source, and the resultant state of overcurrent will give rise to thyristor destruction and other trouble. Thus, in the event of commutation failure of the chopper, this commutation failure must be immediately detected to disconnect the d.c. load from the d.c. power source as quickly as possible for protecting the equipment. In this case, commutation failure of the chopper is detected on the basis of logical judgement of the relation between the on-off gate signal applied to the chopper and the actual on-off operation of the chopper. A protective circuit based on such logical judgement for the purpose of detection of commutation failure of the chopper has the advantage of a very short length of time required for the detection of commutation failure and is employed in electromobiles, battery-operated fork-lift trucks, etc.
Such a commutation failure protective circuit is generally formed on a printed circuit board. Means such as multiconnector terminals are generally used to electrically connect this protective circuit with the main circuit including the d.c. power source, d.c. load and chopper. However, trouble such as malcontact or wire-breaking occurring in the interface between the main circuit and the protective circuit will result in impossibility of detection of commutation failure of the chopper, giving rise to such a very dangerous state as burn-out of the parts including the thyristors or runaway of the vehicle.